The invention relates to a twisted flat cable comprising twisted pair portions and parallel fused portions provided alternately with the twisted pair portions in the longitudinal direction of the cable.
Twisted flat cables comprise a plurality of twisted pairs juxtaposed to each other or one another while providing parallel portions at predetermined intervals. The twisted pair is composed of two conductors that are insulated separately and twisted together. The parallel fused portion is composed of separately insulated conductors that extend from the twisted pairs and are disposed side by side with adjacent insulated conductors being fused to each other. The twisted flat cable is advantageous in that the precision of the conductor-conductor dimension is high and a connector for IDC (Insulation Displacement Contact) can be easily attached.
In conventional twisted flat cables, PVC (polyvinyl chloride) has been used as an insulating material for an insulative layer covering the conductor. The structure of the twisted flat cables is generally such that, in order to flatten the cable, the cable is sandwiched between two plastic tapes.
In recent years, however, the transmission characteristics for electronic signals of the twisted flat cables using PVC as the insulating material have been found to be unsatisfactory as advanced hard disk drive interfaces such as SCSI (small computer system interface). It has been pointed out that the unsatisfactory transmission characteristics are, of course, attributable to poor dielectric properties of PVC per se.
The dielectric properties of PVC are specifically such that the dielectric constant is 3 to 8 and the dielectric loss tangent is 4 to 12%. Both the dielectric constant and the dielectric loss tangent depend greatly upon the temperature and the frequency.
Accordingly, it is an object of the invention to solve the above problem of the prior art and to provide a twisted flat cable possessing excellent transmission characteristics.
According to the first feature of the invention, a twisted flat cable comprises: twisted pair portions; and parallel fused portions provided at predetermined intervals alternately with the twisted pair portions in the longitudinal direction of the cable, said twisted pair portions each being composed of a plurality of twisted pairs juxtaposed to each other or one another, said plurality of twisted pairs each being composed of two conductors, which are separately insulated by an insulative layer formed of an insulating material and are twisted together, said parallel fused portions each being composed of separately insulated conductors which extend from the twisted pair portion and are disposed side by side, adjacent insulated conductors in their respective insulative layers in the parallel fused portion being fused to each other to form a fused portion, said insulating material having a dielectric constant of not more than 2.6 and an oxygen index of not less than 25.
TPO (thermoplastic olefin) is best suited as the insulating material, because TPO can satisfy the above requirements, is stably extrudable, and good fusibility.